1. Field of the Invention
The present invention relates to ink jet printing apparatus and more particularly to improved systems for detecting the level of remaining ink in the supply reservoir of such apparatus.
2. Description of Background Art
In continuous ink jet printing apparatus streams of uniformly spaced ink drops are created by imposing predetermined vibrations upon liquid ink filaments issuing from an orifice plate. The filaments are formed by supplying ink under pressure to a print head cavity that is in communication with the orifice plate. Information is imparted to the droplet streams by selective non-charging or charging and deflection of droplets. A portion of the droplets pass to the recording medium but there are a substantial number of non-printing droplets which are intercepted by a catcher for recirculation. Often the print head cavity has an outlet other than the orifice plate (e.g. to facilitate dynamic pressure control within the cavity at start-up), and the apparatus ink supply system also circulates such ink flow.
In such apparatus, it is highly desirable to detect that the ink supply is at a "replenish-condition" prior to the time that the ink supply becomes insufficient to achieve proper printing operation. Failure to provide such detection could cause spoilage of a considerable amount of print output if the problem is not visually detected. Also operation in such a low ink condition could necessitate a lengthy restart cycle, e.g. in order to remove air from the system, or could even cause machine damage.
Various physical approaches and devices have been used in the prior art to detect ink level in the ink supply reservoir. For example electrical probes or other such detectors can be introduced into the reservoir at a selected level to detect the existence or non-existence of the ink. This approach and other such sophisticated electrical detection schemes are highly useful in systems where the ink reservoir is an integral portion of the printer apparatus.
However, such approach is not so desirable in all applications. As described in U.S. application Ser. No. 722,548, entitled "Ink Cartridge and Cooperative Continuous Jet Printing Apparatus", filed Apr. 12, 1986, in the name of J. McCann, it is desirable that office-use printers have a readily replaceable ink cartridge. That application describes a highly advantageous system wherein a removable cartridge cooperates, with the fluid conduits of a continuous ink jet printer, as the supply/return reservoir for ink circulation. In such a system it is highly desirable that minimum complexity and cost be built into the replaceable ink cartridge.